A Comparative Study of Asexual Reproduction in Three Jellyfish, Aurelia aurita s.1., Chrysaora melanaster and Cyanea nozakii (Cnidaria: Scyphozoa: Semaeostomae), with Special Reference to the Role of Podocysts

The large gelatinous zooplankton such as cnidarian medusae and ctenophores have increased their abundance and caused problematic blooms in many coastal waters worldwide.In particular, in east Asian seas around Japan, China and Korea, where environmental conditions have been increasingly deteriorated by human impact, several cnidarian jellyfish species have become frequently blooming to cause severe damage to fisheries and other human economic sectors.In the light of increased jellyfish population outbreaks or blooms, it is necessary to understand the mechanisms to cause such phenomena.Because it is widely recognized that the asexual reproduction during benthic polyp stage may play a very important role to determine the medusa abundance in the following season, studies dealing with polyps have increased in recent years.However, few studies have been made on the role of podocysts, one of asexual reproduction modes of many semaeostome and rhizostome jellyfish species.The podocyts are a cuticle-covered-cell-mass formed by polyps and they are capable of dormancy until excystment into new active polyps.Therefore, in this study, I aimed to reveal the ecological role of podocysts in three semaeostome jellyfish species, viz.Aurelia aurita s.l., Chrysaora melanaster and Cyanea nozakii, which are frequently blooming not only in Japanese coastal waters but also in Chinese and Korean waters, by conducting mainly laboratory experiments to determine the effects of different environmental factors (i.e.temperature, salinity, dissolved oxygen concentration and food supply) on their encystment, dormancy and excystment in addition to some histological studies.In A. aurita, there are several modes of asexual reproduction by polyps, viz.budding and longitudinal fission, in addition to podocyst formation.The podocysts were never formed by well-fed polyps but only formed by un-fed and poorly-fed polyps (food regime: ≤4.8 µg C polyp - 1 d -1 ), and the podocyst production increased with the increase of temperature.The highest podocyst production during 8-week-experiment (i.e. 6 podocysts polyp -1 ) was attained by un-fed polyp kept at 28 o C. The podocyst production was not affected by salinity within the range from 15 to 32.These results indicate that starvation is a trigger for encystment, while increased temperatures accelerate the encystment rate.The excystment was induced only when podocysts were exposed to cooling temperature (from 28 to 19 o C) and hypoxia (DO: 0.5 mg O 2 l -1 ), indicating that the autumn is the main excystment season.The podocysts were capable of dormancy for up to 3.2 years, but old podocysts (i.e.17-20 months old) excysted rarely (3%).A histological study revealed that newly-formed podocysts contained rich organic reserves (e.g.carbohydrates, proteins and lipids), and the initial reserves may have consumed during dormant period.Only a few nuclei and very weak reaction by RNA were found in the dormant cells of podocysts, indicating that the basal metabolism of the podocysts is low.In Chr.melanaster, the podocyst production is an exclusive form of asexual reproduction by polyps.The production increased with the increase of temperature from 11 to 28 o C. It was lowest by un-fed polyps, and increased with the increase of food supply.A polyp, which was placed in the highest food supply (16.9 µg C polyp -1 ) at 28 o C for 8 weeks, attained the highest production (16.5 podocysts polyp -1 ).Salinity did not affect the podocyst production within the normal range (15-32) where polyps may encounter in the field.The excystment was high (33-48%) when the podocysts were exposed to decreasing temperature (from 28 to 11 o C), but only substantial (6-11%) when they were kept at constant temperatures (18 or 28 o C) and were exposed to increasing temperature (from 18 to 22 or 28 o C).These suggest that the podocysts of this species are capable of excystment without any specific temperature stimuli but attainment higher excystment under cooling temperature condition.Hence, their excystment may mainly occur in autumn.Nearly 100% of podocysts excysted within 12 months even at constant temperatures, indicating that the maximal dormant period is ca. a year.In Cya.nozakii, the podocyst production was also the only means of asexual reproduction and the previously-reported planulocyst formation was not observed.The podocyst production was significantly affected by temperature as well as food supply, both in positive manner, but the production tended to be saturated at temperatures ≥22 o C and at food supply levels ≥4.8 µg C polyp -1 d -1 .The highest podocyst production (6 podocysts polyp -1 ) was attained by a polyp fed with 12.1 and 16.9 µg C polyp -1 d -1 at 26 and 28 o C, respectively.Within the salinity range from 15 to 32, the podocyst production was not affected by salinity.The excystment did not occur at all in the podocysts kept at constant temperatures (18 or 28 o C) and at increasing temperature (from 18 to 22 or 28 o C), but was induced highly (53-65%) when the podocysts were exposed to decreasing temperature (from 28 to 11 o C), indicating that the major excystment period of Cya.nozakii is autumn.The present study has clearly demonstrated that the podocysts play important roles in the seasonal population dynamics of these three semaeostome jellyfish species.The major ecological roles of podocysts may lie into two aspects; one is for a reproduction to increase the polyp population abundance and another is for a refuge to protect the population from unfavorable environmental conditions.As the podocyst production is an exclusive form of asexual reproduction in Chr.melanaster and Cya.nozakii, the above-mentioned two aspects are equally important in their seasonal population dynamics.However, in A. aurita, the podocyts are produced only by starved and scarcely-fed polyps; they may play an important role as a strategy for population refuge to overcome unfavorable environmental conditions.In this species, budding is a common mode of asexual reproduction and the rate of budding is much higher than that of the podocyst production.The type of strobila may also affect the increase of the medusa population abundance; it is poly-disc in A. aurita and Chr.melanaster, and mono-disc in Cya.nozakii.Hence, based on the above-mentioned modes and rates of asexual reproduction by polyps, the potential to increase the medusa population abundance, or to cause jellyfish bloom, is highest in A. aurita, followed by Chr.melanaster and Cya.nozakii.Since these basic biological features specific to each jellyfish species are influenced by environmental conditions (e.g.temperature, food supply and hypoxia), concomitant studies both in the controlled laboratory experiments and in field survey on both environmental variables and polyp and medusa population dynamics are always necessary in order to understand the mechanisms to cause jellyfish blooms. ContentsChapter 1.General introduction Global expansion of jellyfish blooms ……………………………………………...1 Causes of jellyfish blooms………………………………………………………….4Life cycle of scyphozoan jellyfish………………………………………………… 5 Brief overview of podocyst studies………………………………………………...6 Ecological significance of podocysts ………………………………………………8 Objectives of this study……………………………………………………………..8 Chapter 2. Asexual reproduction of the moon jellyfish Aurelia aurita s.l. with special reference to the role of podocysts Introduction………………………………………………………………………....10 Materials and methods………………………………………………………………12 Results………………………………………………………………………………17 Discussion…………………………………………………………………………...22 Chapter 3. Asexual reproduction of the Japanese sea nettle Chrysaora melanaster with special reference to the role of podocysts Introduction………………………………………………………………………….26